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Shrinkage of expansive soils may cause serious problems to geotechnical and geo-environmental structures. In this study, the shrinkage behaviour of compacted clay treated with 4% lime was investigated. The samples of compacted clay without lime treatment were also tested for comparison. Besides, changes in the microstructure were analysed by mercury intrusion porosimetry tests on both treated and untreated samples. The results showed that the shrinkage curve of untreated sample is close and almost parallel to the saturation line, prior to approaching the compacted shrinkage limit (shrinkage limit for compacted soils). By contrast, the shrinkage curve of lime-treated soil is located leftwards and exhibits a higher compacted shrinkage limit, evidencing a clear microstructure effect. Further analysis showed that large aggregates with large pores are formed by lime treatment and these large pores play an important role in the shrinkage behaviour of soils. In addition, the curing time effect seems to be insignificant on the shrinkage behaviour of lime-treated soil.

To clarify the mechanism of stiffness change by pozzolanic reaction for lime-stabilised soils during curing, it is essential to avoid the effect of water evaporation in this process. In this study, changes of small strain shear modulus Gmaxand total suction of a lime-treated silt during curing were investigated, with water content kept constant to avoid the effect of water loss. The soil samples were prepared with 2% lime, and then compacted dry of optimum (17%) and wet of optimum (22%). The Gmaxvalues were determined at various curing times using bender elements and the suction was measured using a hygrometer WP4. Results show that a two-phase development of Gmaxoccurs for the lime-treated samples. The Gmaxincreases slowly in the first phase, corresponding to the flocculation process, but then Gmaxstarts to increase significantly in the second phase owing to the substantial cementitious compounds produced through pozzolanic reaction. Interestingly, the suction changes follow the same two-phase pattern, suggesting that the production of cementitious compounds increases the soil stiffness; it also modifies the soil microstructure, leading to soil water redistribution and thus changes in suction.

International audience; Like bricks of compacted bentonite/sand mixtures, mixtures made up of pellets and powder of bentonite are considered as a possible material to make the sealing plugs used to fill up galleries and ensure long-term watertightness in deep radioactive waste disposal. Pellets/bentonite mixtures have a low permeability, high swelling capacity, good radionuclide retention capability and operational advantages in terms of placement in the galleries. Following a previous in-depth characterisation of bentonite pellets/powder mixture conducted by the same group, an investigation of the water-retention properties and microstructure changes of a bentonite pellet subjected to wetting/drying cycles under free swelling conditions was carried out by means of mercury intrusion porosimetry (MIP) and X-ray microtomography. A complete description of the changes in water content, void ratio and degree of saturation of the pellet was provided. Data showed that the free swelling of the pellet is due to the combined effect of both crack propagation at the macro scale, and the swelling of bentonite grains, governed by hydration mechanisms along the smectite faces at the nano scale. Significant development of a crack network is observed between 38 and 9 MPa. For suctions below 9 MPa, there is a significant decrease of the platelet thickness and an increase in the disorder of the platelet assembly, resulting in the average MIP entrance pore radius increasing to 0·4 μm within the expanded bentonite grains.

Loess covers large portions of Earth's surface. Loess deposits are typically composed of silt with clay and fine sand particles and usually reach a few meters in thickness. Thermal conductivity of loess varies over a relatively large range from 0·07 to 2 W/(m.K), depending on the particle composition, soil texture and also moisture content. In this study, intact loess was sampled at shallow depth (1 m) in Northern France. Suction, volumetric moisture content and thermal conductivity of soil were measured simultaneously during the application of wetting/drying cycles to the sample. The results show that the volumetric moisture content significantly influences the soil thermal conductivity. As is the case for the water-retention curve, a hysteresis loop can be observed when plotting the thermal conductivity against suction. However, no hysteresis appears when the thermal conductivity is plotted as a function of the degree of saturation.

Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. When the compacted bentonite blocks are emplaced, technological voids related to different interfaces involving the buffer material are created, and their hydro-mechanical behaviour is of primary importance for the safety of disposal. In this study, the hydraulic resistance of the interface between compacted MX80 bentonite and Boom Clay is investigated in the laboratory using an injection cell. The results obtained show that when water is injected, the technological gap is quickly reduced due to the bentonite swelling. When water pressure reaches the hydraulic resistance of the interface, hydraulic fracturing takes place with a drastic pressure decrease. After fracturing, water injection continues and bentonite continues to swell. A higher subsequent pressure is needed to produce a new hydraulic fracturing. After a certain time, the hydraulic resistance becomes high enough so that no further fracturing occurs, suggesting that the technological gap is sealed.

An experimental programme was carried out with the aim of investigating suction effects in a specimen of Callovo-Oxfordian claystone extracted at great depth (479 m). A specimen that was saturated in an oedometer with limited swelling exhibited significant swelling and desaturation upon quick unloading at constant water content. The microstructure after unloading was compared with the initial microstructure based on mercury intrusion porosimetry measurements. Since the pore size distribution curves before and after loading appeared to be comparable in the range of pore sizes covered by mercury intrusion (entrance diameter between 363ṡ6 μm and 5ṡ5 nm), it was concluded, based on microstructure considerations, that swelling occurred in pores with a diameter less than 5ṡ5 nm, from water transfer within the clay matrix from inter-platelet to intra-platelet pores. This mechanism might occur when excavating galleries in areas of an excavation damaged zone that support instantaneous unloading sequences, as a first desaturation process prior to subsequent evaporation due to the ventilation of galleries.

A commercial numerical code was used to simulate the mechanical behaviour of energy piles under thermo-mechanical loadings. The thermal load was simply simulated by imposing on the pile volumetric strains calculated from the coefficient of thermal expansion of the material. Simulations were performed for two existing in situ experiments and one experiment performed using a physical laboratory model. Comparison between the simulations and experimental results shows that this decoupling method can satisfactorily predict the experimentally observed phenomena of pile head movement under thermal cycles at various mechanical loads and change of pile axial load along the pile under thermal cycles. The results highlight the important role played by thermal volume change of the pile on the mechanical behaviour of the energy pile under thermo-mechanical loadings.

Lime treatment is a well-known technique to improve the mechanical response of clayey subgrades of road pavements or clayey soils used for embankment. Several studies show that lime treatment significantly modifies the physical and hydromechanical properties of compacted soils. Nevertheless, studies on the scale effect under climatic changes are scarce. Actually, wetting–drying cycles might significantly modify the microstructure of treated soils, giving rise to changes in hydromechanical properties. This modification could be dependent on the size of soil aggregates before lime treatment. In the present work, this scale effect was studied by investigating the stiffness of a compacted lime-treated clayey soil using bender elements. The studied soil was first air-dried and ground into a target maximum soil aggregates size (Dmax). For each aggregate size, the soil was humidified to reach the target water contents wi, then mixed with 3% of lime powder (mass of lime divided by mass of dried soil) prior to the static compaction at a dry density of 1·60 Mg/m3. Two initial water contents (wi = 14 and 18%) and four maximum soil aggregates sizes (Dmax = 0·4, 1·0, 2·0 and 5·0 mm) were considered. After the compaction, the soil specimen (50 mm in diameter and 50 mm high) was covered by plastic film in order to prevent soil moisture changes. The soil stiffness was then monitored at variable time intervals until reaching stabilisation. Afterwards, the soil specimen was subjected to full saturation followed by air-drying to come back to its initial water content. The results show that: (a) the soil stiffness after lime-treatment is significantly dependent on the aggregate size: the finer the aggregates the higher the soil stiffness; (b) the effect of initial water content on the stiffness is negligible; and (c) the wetting–drying cycles seem to slightly increase the soil stiffness in the case of lime-treated specimens and decrease the soil stiffness in the case of untreated specimens. Furthermore, when an intensive drying was applied reducing the soil water content lower than the initial level, the soil stiffness decreased drastically after the subsequent wetting.

International audience; Compacted unsaturated swelling clay is often considered as a possible buffer material for deep nuclear waste disposal. An isotropic cell permitting simultaneous control of suction, temperature and pressure was used to study the thermo-mechanical behaviour of this clay. Tests were performed at total suctions ranging from 9 to 110 MPa, temperature from 25 to 80 °C, isotropic pressure from 0.1 to 60 MPa. It was observed that heating at constant suction and pressure induces either swelling or contraction. The results from compression tests at constant suction and temperature evidenced that at lower suction, the yield pressure was lower, the elastic compressibility parameter and the plastic compressibility parameter were higher. On the other hand, at a similar suction, the yield pressure was slightly influenced by the temperature; and the compressibility parameters were insensitive to temperature changes. The thermal hardening phenomenon was equally evidenced by following a thermo-mechanical path of loading-heating-cooling-reloading.

Extensive investigations have been and are being carried out on a stiff clay from an underground research laboratory located at Mol (Belgium) called Boom clay, in the context of research into deep nuclear waste disposal. Suction effects in deep Boom clay block samples were investigated through the characterisation of the water retention and of the swelling properties of the clay. The data obtained allowed an estimation of the sample initial suction that was reasonably compatible with the in-situ state of stress at a depth of 223 m. The relationship between suction and stress changes during loading and unloading sequences were also examined by running oedometer tests with suction measurements. A rather wide range of the ratio s/sigma 'v (being s the suction and sigma 'v the effective vertical stress) was obtained (0.61 - 1), different from that proposed by Bishop et al; (1974). Finally, the effect of suction release under an isotropic stress close to the estimated sample suction was investigated. A slight swelling (1.7%) was observed and further compression provided a satisfactory value of the overconsolidation ratio confirming the suggestions of taking some precautions before putting a swelling sample in contact with water as suggested by Graham et al. (1987). The various experimental data gathered in this study finally evidenced a relatively good state of conservation of the block sample used.